The present invention relates to a fine grained cemented carbide containing chromium. By adding small but well controlled amounts of Ti, Nb, V, Zr or Ta alone or in combinations, a further grain refined cemented carbide structure has been obtained without embrittling secondary phases.
Cemented carbide cutting tools with a grain refined structure are used today to a great extent for the machining of steel, hardened steel, stainless steels and heat resistant alloys in applications with high demands on both toughness and wear resistance. Another important application is in microdrills for the machining of printed circuit board, so-called PCB-drills. In these types of applications it is known that the amount and size of defects, such as abnormal WC-grains, small precipitates of embrittling phases, porosity, binder phase lakes, are critical for the mechanical properties, e.g. fracture toughness and transverse rupture strength (TRS).
Common grain growth inhibitors include vanadium, chromium, niobium, tantalum or compounds involving these. When added, generally as carbides and in an amount of from about 0.15 to about 1.5 wt-%, they limit grain growth during sintering, but they also have undesirable side effects, affecting the toughness behaviour in an unfavourable direction. Vanadium is known to be the most effective grain growth inhibitor but is also particularly detrimental due to its negative influence on the toughness properties. Normal levels of vanadium additions are also known to result in precipitation of embrittling phases in the WC/Co grain boundaries.
In order to achieve a fine, as sintered grain size of less than about 0.6 μm for cemented carbides with Cr/Co-ratios of from about 0.04 to about 0.06, very fine WC raw material is needed and the sintering is generally performed at a relatively low temperature, e.g. close to a temperature of about 1360° C. and often by sinterHIP or followed by hot isostatic pressing in order to obtain a dense structure. Such a production route of course increases the production cost, but also increases the risk of uneven binder phase distribution. In addition, the use of these very fine WC-raw materials gives powders with inferior compaction properties, resulting in powders that are difficult to compact to complicated geometries such as cutting inserts.
According to WO 99/13120, the amount of grain growth inhibitors can be reduced if a carbon content of the cemented carbide close to eta-phase formation is chosen.
JP-A-11-152535 discloses a process to manufacture fine grained tungsten carbonitride-cobalt hard alloys using tungsten carbonitride as a raw material. JP-A-10-324942 and JP-A-10-324943 disclose methods to produce fine grained cemented carbide by adding the grain growth inhibitors as nitrides. In order to avoid pore formation by denitrification of the nitrides sintering is performed in a nitrogen atmosphere.
EP-A-1500713 discloses a method of making a fine grained tungsten carbide-cobalt cemented carbide comprising mixing, milling according to standard practice followed by sintering. By introducing nitrogen at a pressure of more than 0.5 atm into the sintering atmosphere after dewaxing but before pore closure a grain refinement including reduced grain size and less abnormal grains can be obtained.
EP-A-1803830 discloses cemented carbides which include WC having an average particle diameter of <0.3 μm as a hard phase and 5.5 to 15 wt-% of at least one iron group metal element as a binder phase, and comprise, in addition to the above hard phase and binder phase, 0.005 to 0.06 wt-% of Ti, Cr in a weight ratio relative to the binder phase of 0.04 to 0.2. Especially, the above cemented carbides contain no Ta.
According to US 2006/029511, there is provided a method of making a fine grained tungsten carbide-cobalt cemented carbide comprising mixing, milling according to standard practice followed by sintering. By introducing nitrogen at a pressure of more than 0.5 atm into the sintering atmosphere after dewaxing but before pore closure a grain refinement including reduced grain size and less abnormal grains can be obtained.